Recently, optical communication networks utilizing the broadband characteristics of an optical fiber are gaining attention. For example, optical access networks where optical fibers are laid for each house as with FTTH (Fiber To The Home), for supplying multi-media services such as CATV or VOD are gaining considerable attention.
FIG. 1 shows a construction of an ATM-PON (Passive Optical Network) that has gained attention as one of systems for realizing an optical access network.
In FIG. 1, the ATM-PON has a basic structure where a plurality of subscriber side apparatus #1–#n are connected via an optical coupler to a single station side apparatus connected to an ATM network. In such an ATM-PON, in a case where data (upstream data) is transmitted from each of the subscriber sides to the station side, a burst transmission technique is required. For this burst transmission technique, in the transmission section of each of the subscriber side apparatus, as well as requiring a high extinction ratio compared to the conventional trunk line system, preferably the “0” level optical output is as close as possible to non emission.
More specifically, as shown for example in FIG. 2A, for respective optical outputs D1–D3 after being respectively transmitted from the subscriber side apparatus #1–#3 and multiplexed by the optical coupler, the minimum value L1min of the “1” level (in the figure, the “1” level of optical output D2) in all of the optical outputs D1–D3 must exceed the maximum value L0max of the “0” level (in the figure, the “0” level of the optical output D3), and the minimum value L1min of the “1” level must not become less than the maximum value L0max of the “0” level as shown in FIG. 2B.
For the conventional driving method of a semiconductor laser (LD) on the subscriber side, for example, as shown by the current-light output (I-L) characteristic for a LD in FIG. 3, there is a method where in all of the subscriber side apparatus, the LD current of the “0” level is set to 0 mA, or to a system allowable minute current.
Furthermore, in a driving method described for example in Japanese Unexamined Patent Publication No. 61-80922, there is disclosed a method where, as shown by the I-L characteristic in FIG. 4, a bias current is supplied to the LD immediately before and immediately after an occurrence of optical output of the “1” level. Moreover, in the driving method described for example in Japanese Unexamined Patent Publication No. 9-83050 filed by the present applicant prior to the present invention, there is disclosed a method where, as shown in FIG. 5, a bias current is supplied to the LD only in a period immediately before transmission of burst data (for example, a fixed length cell packet) and during the transmission. Such a driving method adopts a so-called pre-bias method where the bias current is supplied according to the transmission period of the burst data, and in the non-transmission period, the bias current supply is not performed.
In addition, in the driving method described for example in Japanese Unexamined Patent Publication No. 61-131631, there is disclosed a method which, in order to shorten the drive time for the LD at the time of starting burst data transmission, supplies a minute fixed bias current to the LD also in the data non-transmission period, and uses the abovementioned pre-bias current and fixed bias current together.
However, in the abovementioned known driving methods, there are the following problems.
In the driving method shown in FIG. 3, if an oscillation delay time of the LD is considered, there is a possibility that this method cannot cope with a high transmission speed. In the current situation, it is a limit for this method to cope with a transmission speed of around 100˜200 Mb/s.
Furthermore, in the driving method shown in FIG. 4, the higher the transmission speed becomes, the more difficult it becomes to accurately and reliably supply the pre-bias current immediately before and immediately after generating an optical output of “1” level. More specifically, the realization of a circuit or the like for determining the timing for the pre-bias current is difficult.
Moreover, in the driving method shown in FIG. 5, there is a possibility that the start-up time at the time of starting the supply of pre-bias current causes a problem. Unless the start-up is performed at a high speed, this method becomes equivalent to the driving method shown in FIG. 3, and an effect of this driving method cannot be sufficiently obtained. For example, it is considered that if a large capacity LD is used, the start-up at a high speed will be difficult.
In addition, in the driving method that uses the pre-bias current and the fixed bias current together, it is likely that the time required when converting from the pre-bias current to the fixed bias current after transmission of burst data, will cause a problem. More specifically, as with the description in the abovementioned publications, if the change of the bias current is to be realized by discharge of a capacitor, then in the case where the transmission speed is a high speed, there is a possibility that the change of the bias current is not converged to within 1 bit of data. Furthermore, it is necessary to apply from outside a signal (for example, an enable signal or the like accompanied the burst data) for judging the transmission period and the non-transmission period of the burst data. Therefore, there is also the drawback that it is difficult to flexibly cope with the various transmission systems.